The present invention relates to power supplies and more particularly to a power supply having a transformer for converting mains power into AC or DC power.
A typical power supply includes a transformer. A transformer is an electrical device that transfers electrical energy from a primary coil through a time varying magnetic field to a secondary coil upon the addition of an electrical load to the secondary coil. Transformers are used in a variety of applications, such as mains power delivery for electronics and children's toys, and come in a variety of sizes, from very large to miniature.
A conventional transformer consists of a core, which may be made of iron or some other ferrous material, in the shape of a loop. On one side of the core, wire is coiled around the core, and primary or mains voltage is applied. This side of the core is called the primary. Opposite the primary is a similar arrangement called the secondary. By varying the number of turns of wire in the coil more explicitly the turns ratio of the primary to secondary, the voltage may be stepped up or down depending upon the required usage, and the load applied to the secondary.
One disadvantage of transformers using an iron core is that to achieve the necessary performance, the core had to be of a substantial size and requisite weight. This results in transformers that are bulky and difficult to package. In many cases these bulky transformers consume precious space, such as those transformers built directly into the mains plug, such that the transformer blocks other outlets, as can be the case in power strips or wall outlets. Due to the weight of traditional transformers, additional effort must be expended to insure that that transformer casing is suitably strong, to withstand a drop which may occur during normal use. If the transformer is to be wall mounted, certain applications may not be suitable due to the excess weight placed upon wall mounts.
Also, the cost of the iron in traditional transformers is also an undesirable factor. In addition to the direct material cost of iron or other ferrous metals, the traditional transformer must be made larger to accommodate the large coil, necessitating the use of additional other materials, such as plastic, increasing cost. Copper wire or other conductive material is used abundantly in forming the primary and secondary coils, further adding to the cost of traditional transformers.
Further, the iron core inherently retains a sizeable buffer of energy, which is capable of being discharged through a short circuit. Because energy is stored in the iron core, in the event of a short circuit, it can take considerable time for the energy to dissipate from the system, which could cause damage. Further, the transformer may continue to provide power either until the mains power breaker trips or the transformer itself fails, or the short circuit is removed.
Traditional transformer power supplies are not dimmable, as a result of their fixed turns of wire that only step up or step down an input voltage for a set output voltage. As a result, if dimming is required, a power supply with a transformer is not a desirable selection as a power supply. For example, in the case of lighting, there are many applications where full light intensity is not desired at all times, as in the case of security lighting. A security light could be set to run at less than full power for normal operation, then switch to full power if a motion sensor connected to the system detects motion, for example. This operation would conserve energy yet also provide illumination for appearance and security. Unfortunately, a power supply with a traditional transformer is not usable in this application.
Voltage drop over a distance has plagued power supplies with traditional transformers. Using the example of landscape lighting, a transformer is located near a mains power source, and a power supply line is connected to it. The line runs from the power supply until it terminates some distance away, for example, 50 feet. Lights are provided that clip into the supply line using connectors that pierce the wire to make an electrical connection. In this way the position of the lights can be varied according to the particular landscape application. The power supply line, like any wire, has some resistance. So the voltage measured at the end of the supply line will be lower than at a point located near to the transformer. With a power supply having a traditional transformer, the instructions recommend placing the lights at somewhat equal distances along the supply line in order to compensate for the resistive effect of the wire. However, the particular landscape application may call for most or all of the lights to be installed toward the end of the supply line. A power supply with a traditional transformer may have difficulty adequately powering the lights, and the lights at the very end may be dim or fail to light at all. This situation is undesirable and places unnecessary limits on landscape lighting, or other applications.
Yet another disadvantage of an iron core transformer is the inability to compensate for fluctuations in mains voltage. If there is a power spike or sag, the iron core transformer is not equipped to protect the devices it is powering, which could result in permanent damage. No logic or circuitry is present in a power supply with a traditional transformer, to detect power fluctuations. Since many modern electronic devices are sensitive to such power fluctuations, use of a traditional transformer with these devices could result in damage or destruction of these devices.